1. Field of the Invention
The present invention relates to an exhaust gas purification system, and in more particular, to an exhaust gas purification system for purifying a NOx in an exhaust gas exhausted from a NOx exhaust source such as a lean-burn engine and a gasoline direct-injection engine under an excess oxygen atmosphere.
2. Description of the Related Art
Conventionally, as a system for purifying nitrogen oxides (hereinafter referred to as NOx) even under a condition where an oxygen concentration is high like an exhaust gas of a diesel engine, an exhaust gas purification system using a NOx adsorptive catalyst and another one using an urea-selective catalytic reduction (Urea-SCR) are known. However, the exhaust gas purification system using the NOx adsorptive catalyst has a problem that a huge fuel consumption loss occurs because it needs to change an air-fuel ratio of an engine from lean to rich and further to stoichiometric. On the other hand, in the exhaust gas purification system using the urea-selective catalytic reduction, a problem remains in a point that a development of infrastructure of an urea becomes indispensable.
Consequently, to solve these problems, systems using NOx selective reduction catalysts, to be more precise, a platinum catalyst (for example, from a 14th row in a fifth column to a 25th row in a sixth column of page 3 of Japanese patent 2909553), an iridium catalyst (for example, from a fourth row in a fifth column to a 20th row in a sixth column of page 4 of Japanese patent laid-open publication Hei 6-31173), and a silver catalyst (for example, from a 35th row in a sixth column of page 4 to a 22th row in an eighth column of page 5 of Japanese patent-laid open publication Hei 5-92125) instead of the NOx adsorptive catalyst and urea-selective catalytic reduction are proposed.
However, in an exhaust gas purification system using the platinum catalyst if a reducing agent, for example, a hydrocarbon (hereinafter referred to as HC) is not added to an exhaust gas, a NOx purification ratio becomes low, and if an added amount of the reducing agent is increased, a temperature of the NOx selective reduction catalyst becomes out of a range of a purification temperature region due to oxidizing heat, so a system with a high purification ratio cannot be built.
In addition, in an exhaust gas purification system using the iridium catalyst, a NOx purification temperature of the catalyst is high and moreover the catalyst cannot be said to be sufficient in a selectivity for a paraffin. Accordingly, in the exhaust gas purification system an exhaust gas temperature is low, so if it is applied to a diesel engine of which paraffin concentration is high in an exhaust gas, it cannot sufficiently purify the NOx in the exhaust gas.
In addition, in an exhaust gas purification system using the silver catalyst, a NOx purification temperature of the catalyst is high, so if it is applied to a diesel engine of which exhaust gas temperature is low, it cannot sufficiently purify the NOx in the exhaust gas.
Relating to such the technologies, “Plasma-Assist Catalyst for Cleaning NOx” in which a plasma reactor and silver/alumina catalyst are combined is proposed (for example, see pages 2 to 5 and FIG. 1 in Japanese patent laid-open publication 2002-210366).
However, in “Plasma-Assist Catalyst for Cleaning NOx” disclosed in Japanese patent laid-open publication 2002-210366, there exists a problem that when adding the HC and the like as a reducing agent to maintain the NOx purification ratio, caulking occurs and thereby the NOx purification ratio is lowered with a change over time. As one of its causes, as shown in FIG. 11, it is thought that the HC and the like invade in a recessed position of narrow porosities 124 of a NOx selective reduction catalyst layer 120 stacked on a support body 111. The HC and the like are then self-collected on surfaces of porous supports 121, and thereby become incrustations 126 with which NOx selective reduction catalysts 122 such as silver supported on the surfaces of the porous supports 121 are coated.
Thus, since the incrustations 126 damage a catalytic function of the NOx selective reduction catalysts 122 and decrease a contact area with an exhaust gas finally occluding the narrow porosities 124, there exists a problem that the NOx purification ratio becomes lowered. Meanwhile, a phenomenon that the NOx purification ratio thus becomes lowered due to the incrustations 126 is called “caulking.”
In addition, as a configuration of another conventional exhaust gas purification system is known such one where an HC adding device, a plasma reactor, a purification device having a NOx selective reduction catalyst are provided with an exhaust pipe of an diesel engine from its NOx exhaust resource side in this order and thereby purifies the NOx in an exhaust gas. The HC adding device is a device to add the HC as a reducing agent to the exhaust gas since an HC amount is few in the exhaust gas of the diesel engine, and in the purification device a catalyst where a zeolite is made to support active compositions such as Cu, Co, and the like is used (for example, see pages 2 to 5 and FIG. 1 in Japanese patent laid-open 2002-210366).
When using the zeolite as a support as described above, the zeolite partially oxidizes the HC of a reducing agent and produces an active CHO with a strong reducing ability by an oxidizing ability of the active compositions and an acid titer of the zeolite, thereby the NOx being able to be reduced and purified by the active CHO. Since even if a temperature of an exhaust gas is low, the CHO is produced, the zeolite is suitable for an exhaust gas treatment of the diesel engine.
However, since in the zeolite an adsorption of the HC tends to precede a partial oxidization of the HC and the zeolite is added comparatively more in amount taking its reactivity into consideration, the adsorption amount of the HC mounts. Therefore, when an exhaust gas temperature is low, an adsorbed HC is not separated, and resultingly, there exists a problem that an occurrence of caulking could cause a lowering of the NOx purification ratio.
Accordingly, an exhaust gas purification system of which NOx purification ratio is higher is strongly requested.
In addition, an exhaust gas purification system that is designed to be able to maintain a higher NOx purification ratio over a long period by restraining the occurrence of the caulking in the zeolite when using it as a support is also requested.